The defining neuropathological lesions of AD are the deposition of amyloid beta (Abeta) in the form of amyloid fibrils in congophilic angiopathy and senile plaques, as well as the presence of neurofibrillary tangles. The Abeta in neuritic plaques is predominantly 1-42, while in vessels it is mainly Abeta1-40. Preamyloid lesions, the earliest types of Abeta deposits, are mainly Abeta17-42. Abeta peptides are also found in all biological fluids, with a main sequence of Abeta1-40. The latter is called soluble Abeta (sAbeta). Since Abeta peptides are produced throughout the body and can cross the blood brain barrier (BBB) in both directions, it remains unknown why amyloid deposition in AD occurs only in the brain. In this grant we will test the hypothesis that the clearance of brain sAbeta and its deposition is significantly affected by its binding proteins, such as apolipoprotein (apo)E. In the plasma sabeta is thought to be bound to apoJ, albumin or transthyretin. On the other hand, we have preliminary evidence that brain sAbeta in AD patients is partially bound to apoE, a protein that does not cross the BBB. Previously we have reported that in neuritic plaques amyoid Abeta is partially complexed to a carboxyl fragment of presenilin-1, a protein which is linked to the majority of early onset familial AD. In this grant we will determine how apoE and other Abeta peptide binding proteins interact with Abeta1-42, 1-40 and 17-42, to influence their passage across the BBB from the brain, as well as their conformation, aggregation state, toxicity in tissue culture and their ability to bind to senile plaques in vivo. We plan to: 1) Determine how much sAbeta from brain tissue is complexed to apoE and other proteins in normal controls versus AD and DS patients of differing ages and apoE isotopes. This will ascertain the importance of brain sAbeta-apoE complexes in AD pathogenesis. 2) Determine the influence of apoE and other sAbeta binding proteins on the conformation of Abeta1-42, 1-40 and 17-42 using FT-IR, circular dichromism and other methods. The influence of these binding proteins in tissue culture will be determined. 3) Determine if labeled Abeta1-40, 1-42 and 17-42 alone and with different binding proteins can cross the BBB either from ventricular or systemic injections in a transgenic model of AD, as well as in aged monkeys, with vessel amyloid and parenchymal Abeta deposits. We will also identify if any of these labeled peptides are deposited on the Abeta lesions, in vivo. The latter could be used to develop a diagnostic test for AD.